This work was supported by EPSRC grant number RG5560

Bain, CD

Berson, A

Brown, PS

Talbot, EL

English

Apollo (Cambridge)

Talbot, E.L., Berson, A., Brown, P.S. and Bain, C.D., paper NIP28 2012,'Drying and Deposition of Picolitre Droplets of Colloidal Suspensions in Binary Solvent Mixtures' (accepted) Drying and Deposition of Picolitre Droplets of Colloidal Suspen-sions in Binary Solvent MixturesE.L. Talbot1, A. Berson2 and C.D. Bain1; 1Department of Chemistry, Durham University, Durham, DH1 3LE, United Kingdom; 2Schoolof Engineering and Computing Sciences, Durham University, Durham, DH1 3LE, United KingdomAbstractPicolitre droplets of colloidal suspensions in a mixture of twosolvents are emitted by a drop-on-demand ink jet print head ontocoated and uncoated glass substrates. The evaporation rate andinternal flows inside the drying droplets are investigated and thedeposit pattern formed is related to the drying dynamics.High-speed imaging of the droplet profile from the side al-lows the droplet diameter, height, contact angle and volume to bemeasured during evaporation. The internal flows throughout dry-ing are visualized by following tracer particles within the fluidwith an inverted microscope. The resulting deposits at highersolid content are imaged by scanning electron microscopy in or-der to relate the morphology and fine structure to the internalflows within the droplet.The evaporation of binary solvent mixtures can cause a gra-dient in the surface tension at the liquid-air interface, resultingin a Marangoni flow. The ratio of solvent composition and thesurface tension of the more volatile solvent relative to the lessvolatile solvent is varied, to manipulate the direction and mag-nitude of any introduced Marangoni flow. Pure solvent dropletsare compared to mixed binary solvent systems, to determine theimportance of Marangoni flows on the morphology of the finaldeposit.IntroductionThe ring deposits left after colloidal droplets dry are unde-sirable for ink jet printed graphics and printed electronics, wherea homogeneous deposit is needed. The “coffee ring” stains de-velop in colloidal droplets drying with a pinned contact line [1].Self-pinning of colloidal droplets is common, particularly for highsolids content where particles wedge at the contact line prevent-ing retraction. In order to conserve mass, a radial flow towards thecontact line replenishes evaporated liquid. This convective flowtransports particles inside the droplet to the periphery, building upa ring stain. One route to countering these rings is to introduce aMarangoni flow opposing the radial convective flow [2, 3].Solvent mixtures can exhibit markedly different internalflows and evaporation behavior to their pure component fluids[4, 5]. Mixtures can be used to introduce a re-circulation flow,as preferential evaporation of the more volatile solvent causes de-pletion of that component at the contact line compared to the apex[7]. The resulting surface tension gradient across the free surfaceof the droplet causes fluid to flow from low to high surface ten-sion. The magnitude of the Marangoni flow increases as the sur-face tension gradient across the liquid-vapor interface increases.We use ethylene glycol-water and ethanol-water mixturesloaded with polystyrene spheres to determine the influence ofMarangoni flows on the end deposit. A trace amount of particles isincluded to visualize internal flows within the droplet. The flowsare then linked to the deposit structure resulting from a highersolid content using scanning electron microscopy.ExperimentalSingle picolitre droplets were emitted from a drop-on-demand Microfab print head (MJ-ABP-01, Horizon Instruments),controlled by a Microfab JetDrive III Controller. The nozzleorifice was 50 µm. Fluids consisted of mixtures of ethanol-wateror ethylene glycol-water filtered through a 0.45 µm pore filter.These mixtures were chosen as both ethanol and ethylene glycolhave surface tensions lower than water, but ethanol is morevolatile and ethylene glycol is less volatile than water (table 1).Therefore, any Marangoni flows should flow along the interfacefrom the apex to contact line for ethanol-water and from contactline to apex for ethylene glycol-water mixtures.Table 1. Fluid surface tensions, σ and vapor pressures, P, at20◦C [6].Fluid σ\ mNm−1 P\ kPaEthanol 22.39 5.9457Ethylene glycol 48.43 0.0074Water 72.88 2.3374Tracer particles of 620-nm diameter polystyrene spheres(Bangs Laboratories) were included at 0.01%v to follow internalflows. Separately, 220-nm diameter spheres (Leeds University)were added to the mixed solvents at 1%v solids, in order to viewend deposits by scanning electron microscopy (SEM). Ethanolsolutions with 1%v solids dried in the nozzle, so pure ethanolwith 1%v solids was not printed. Consequently, in order to printethanol-water solutions with 1%v solids, the humidity at the noz-zle was raised to prevent clogging. This treatment was not neededfor ethylene glycol solutions, as ethylene glycol is a humectant.Droplets were deposited onto coated and uncoated glasssubstrates. Ethylene glycol mixtures were deposited onto un-coated glass and ethanol mixtures onto coated glass to increasethe contact angle, improving side-on image acquisition. Glassslides were coated with silane self-assembled monolayers by va-por deposition of octadecyltrichlorosilane (OTS). The slides werefirst cleaned in chromic acid and rinsed with high purity water(MilliQ). Then the cleaned slides were placed in a vacuum desic-cator with an open vial of OTS and the desiccator was evacuated.After 24 hours the slides were removed, rinsed in ethanol anddried in nitrogen.Two experimental set-ups were used to capture images witha high-speed camera (Photron APX RS) from the side and frombelow. To visualize internal flows, droplets were imaged from be-low on an inverted microscope. Droplets were illuminated fromabove at an angle to collect only light scattered from the parti-cles. A 50× magnification objective (Olympus LMPLFN, NA0.5, WD 10.6 mm) was used to magnify images. Illuminationfrom a cold LED light source (Beaglehole Instruments) preventeduneven heating across the droplets. The ambient temperature wasmeasured with a thermohygrometer (Extech) and was kept be-tween 19-23◦C. The relative humidity of the room was 45-50%.Evaporation rates were determined by imaging the dropletsfrom the side. A 20× magnification objective (Nikon MPlan, NA0.4, WD 10 mm) was used, along with the same light source, am-bient temperature and humidity. Images were post-processed inMATLAB to measure the diameter and height of the droplets bydetermining the total number of pixels along the droplet baselineand in the vertical direction respectively. The volume, V , and con-tact angle, θ , were calculated assuming a spherical cap asV =πh6(3R2 +h2)(1)andθ = 2tan−1(hR), (2)where R is the radius of the contact area and h is the apex height.This approximation is valid for picolitre droplets which haveBond numbers, Bo ≪ 1, so are not deformed by gravity.Results and DiscussionDeposit StructureSEM images of ethylene glycol-water deposits containing1%v 220-nm spheres are presented in Fig. 1.Figure 1. Deposits from 10− 90%v ethylene glycol-water, containing 1%v220-nm spheres. Magnification varies and scale bars are 20 µm.Ring deposits were observed for 90%v, 70%v and 50%vethylene glycol content. The 10%v and 30%v ethylene glycol towater ratios gave more uniform deposits, with a thicker raisededge for the 30%v ratio. Deposits from pure water or ethyleneglycol gave rings.Figure 2. Center (i) and contact line (ii) sections for a) 90%v, b) 70%v, c)50%v, d) 30%v and e) 10%v ethylene glycol content, containing 1%v 220-nmspheres. Scale bars are 2µm.The fine structure of ethylene glycol-water deposits at thecontact line and interior are compared in figure 2. The ethyleneglycol-water deposits were mostly a monolayer, with a secondlayer starting for 10−50%v ethylene glycol. The close-up of thecontact line revealed that the outer edge remained a monolayereven for ethylene glycol concentrations above 50%v, where thedroplet height at the periphery was too low for spheres to stack.Interior packing was less dense for higher ethylene glycolcontent. Ratios of 50%v ethylene glycol to water and above gavering deposits, with the interior displaying unfilled regions above70%v ethylene glycol. The 10%v ethylene glycol content gavefull coverage within the deposit.Table 2 shows normalized ring widths for ethanol-water andethylene glycol-water deposits. All ethanol concentrations gavering deposits, with the thickest ring for 90%v ethanol comparedto the deposit diameter.Table 2. Deposit ring widths, w, normalized by the radius, R.The component volume is of ethanol or ethylene glycol (EG).%v component 0 10 30 50 70 90 100Ethanol, w/R 0.16 0.10 0.11 0.20 0.22 0.40 -EG w/R 0.11 1.00 0.55 0.27 0.19 0.29 0.14Evaporation of Mixture ComponentsViewing droplets from the side showed that the initial evap-oration rate was predominantly that of the more volatile compo-nent and towards the end of drying the less volatile componentremained. This is consistent with the work of Sefiane [7].The difference in evaporation rates is more obvious for ethy-lene glycol-water compared to ethanol-water mixtures due to thelarger difference in component vapor pressures. A 50%v ethy-lene glycol mixture is exemplified in Fig. 3 and a 30%v in Fig.4. There is initially a large volume loss for low ethylene glycolconcentrations, with quick evaporation of mainly water, followedby a long period of ethylene glycol evaporation. Less pronouncedpreferential evaporation is observed for ethanol mixtures.Figure 3. An evaporating droplet of 50% ethylene glycol with no solids,at the start of drying. Initially, evaporation was mainly water with a largevolume decrease until 1.86 s. Then ethylene glycol evaporation dominateduntil drying was complete at 232.1 s.0 0.2 0.4 0.6 0.8 100.20.40.60.81t/tdV/Vmax  MainlyWater MainlyEthylene  GlycolFigure 4. Normalized volume loss with time for a 30%v ethylene glycol-water mixture, where td is the drying time.Internal FlowsBy following the progression of spheres during drying, thewhole picture from developing flows inside the droplet to the endresulting pattern can be traced. For pure water droplets, parti-cle motion is radial towards the contact line following evapora-tion driven flow. Comparatively, ethylene glycol-water dropletsinitially have relatively fast particle motion radially towards thecontact line due to the preferential evaporation of water. Particlemotion then slows as the ethylene glycol component dominates,but motion remains radially outwards. Fig. 5 shows particle tra-jectories in a droplet of 10%v ethylene glycol during drying.Figure 5. Particle trajectories for 10%v ethylene glycol-water containing0.01%v 620 nm spheres. a) Motion of particles was initially relatively fasttowards the contact line (0–1 s). b) Radial motion slowed as ethylene glycolbecame the main component (1–2 s). c) Radial motion during remainder ofdrying (2 s onwards). The scale bar is 20µm.For high ethylene glycol content there was no evidence ofMarangoni recirculation preventing radial flow to the contactline. This was expected as the corresponding deposits showedrings, and the surface tension gradient was lower at higherethylene glycol content. Particle motion was slow and radiallyoutwards throughout drying, particularly for 90%v ethyleneglycol. For low ethylene glycol content, i.e. 10%v ethyleneglycol, there was initially a faster radial flow towards the contactline. The uniform deposit at low ethylene glycol content is notfully explained by internal flows, and could be due to particlesnot having time to reach the contact line after the transition toethylene glycol dominated evaporation, along with the increasedsolid concentration from volume loss.The particle distribution within droplets of 50%v and 90%vethanol is shown in Fig.s 6 and 7. The focus is just above thesubstrate (not all particles remain in focus throughout drying).For 10-50%v ethanol-water, the particles were initially distributedthroughout the entire droplet, with motion towards and away fromthe contact line. As evaporation progressed (mainly ethanol), theparticles became restricted to the central region. At the transitionto a water droplet (with residual ethanol), particles were situatedat the center. The particle motion then slowed, and radial motionto the contact line was observed due to evaporation driven flow.For 90%v ethanol, particle velocities were slower, with cir-culation towards and away from the contact line. Unlike the10-50%v cases, there was less restriction of particles to the cen-ter, and no distinct change to water dominated evaporation. The70%v ethanol-water mixture was transitional between these tworegimes, with particles initially distributed throughout the dropletand intermediate restriction to the droplet center.For ethanol mixtures above 50%v ethanol, Marangoni flowwas not strong enough to gather particles at the droplet center.Figure 6. Inverted microscope images for 50%v ethanol-water with 0.01%v620-nm spheres (white). Particles were initially distributed throughout theentire droplet. As evaporation progressed particles were restricted to thecenter. Times are fractions of the drying time, td . The scale bar is 20µm.Figure 7. Inverted microscope images for 90%v ethanol-water with 0.01%v620-nm spheres (white). Particle motion was slower for 10%v ethanol. Theparticles moved towards and away from the contact line in a cycling motion.Times are given as a fraction of the drying time, td . The scale bar is 20µm.At high ethanol content (90%v), slower particle motion indicatedless Marangoni recirculation than for lower ethanol content, witha ring forming in the dried droplet of 90%v ethanol containing0.01%v 620-nm spheres. The thicker ring for the 90%v ethanoldroplet containing 1%v 220-nm spheres suggests early pinningand enhancement of the ring deposit.For mixtures of 10-50%v ethanol with 0.01%v solids, fastMarangoni flow was seen while droplets still contained ethanol.Despite the direction of Marangoni flow in ethanol dropletsenhancing convection, these flows prevented particles gatheringat the contact line, instead collecting particles at a centralstagnation point. The combination of larger particle velocitiesand a higher contact angle at low ethanol content could explainwhy particles were not trapped at the periphery.After the transition to the water dominated regime, particlesprogressed radially towards the contact line, following convectiveevaporation-driven flow. For lower ethanol content the waterdominated regime had longer duration, hence particles had longerto reach the contact line and form a ring. Normalized ring widthsfor 10%v and 30%v ethanol were slightly lower than for water,suggesting a reduction of the ring due to particle motion inwardsduring the start of drying.In addition, ring widths were compared relative to the solidcontent at the transition. It was assumed that the transition for a30%v ethanol droplet was when the droplet volume reached 70%.At this point, the initial solid content of 1%v became 1.4%v.Exponents from a double logarithmic plot of the normalized ringwidth (table 2) against the solid volume fraction were 0.53±0.1for ethanol-water and 0.78±0.1 for ethylene glycol-water. Thesecompare within error to the exponents found for single fluiddroplets [1, 8] of 0.50 for pinned droplets and 0.76 for de-pinningdroplets. This suggests that while Marangoni flow can collectparticles at the center, the solid volume fraction at the transitionto a single component fluid determines the ring width.ConclusionAt low ethylene glycol content (10-30%v), uniform depositswere observed, with rings forming at higher concentrations. Theinternal flows in ethylene glycol mixtures showed relatively fastradial progression of spheres during water dominated evapora-tion, and slow radial motion thereafter with no Marangoni flow.Ethanol-water deposits were all rings. Internal flows showed par-ticle collection at the center for 10-50%v ethanol, during the ini-tial drying phase, until water dominated evaporation gave radialprogression to the contact line. At 90%v particle motion wasslower, with easy pinning due to the lower contact angle, anda ring deposit formed even at low particle concentration. Com-parison of the solid content once a single component fluid wasreached, with the normalized ring width indicated that ring buildup occurred mainly after the transition to a pure fluid.AcknowledgmentsThe authors are grateful to J.P.S. Badyal for his advice onsubstrate fabrication and to H.N. Yow and S. Biggs (Leeds Uni-versity) for providing the 220-nm spheres. This work was sup-ported financially by EPSRC under grant number EP/H018913/1.References[1] R. Deegan, Phys. Rev. E 61, 1, 475–485 (2000).[2] H. Hu and R.G. Larson, J. Phys. Chem. B Lett. 110, 14, 7090–7094(2006).[3] J. Park and J. Moon, Langmuir 22, 8, 3506–3513 (2006).[4] S.M. Rowan, M.I. Newton, F.W. Driewer and G. McHale, J. Phys.Chem. B 104, 34, 8217-8220 (2000).[5] J.R.E. Christy, K. Sefiane and E. Munro, J. Bionic Eng. 7, 4, 321-328(2010).[6] Lange’s Handbook of Chemistry 15th Edition, McGraw Hill (1999).[7] K. Sefiane, L. Tadrist and M. Douglas, Int. J. Heat Mass Tran. 46, 23,4527-4534 (2003).[8] Y.O. Popov, Phys. Rev. E 71, 3, 036313 (2005).Author BiographyEmma Talbot received her MSci Joint Honors in Chemistry andPhysics from the University of Durham, UK in 2010. She then contin-ued at the University of Durham, and is currently working towards a PhDin chemistry supervised by C.D. Bain, based on ink jet printed droplets.

Drying and Deposition of Picolitre Droplets of Colloidal Suspensions in Binary Solvent Mixtures

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